The big end of a connecting rod must be designed so as to incorporate an adequate bearing diameter, at the same time preferably being of such dimensions, when the bearing cap is removed, as to allow the connecting rod to pass through the cylinder bore.
The design of reciprocating piston internal combustion engines has incorporated various means of achieving the largest possible big end bearing diameter consistent with a connecting rod that would pass through the cylinder bore. The advantages of doing so are that firstly for an adequate bearing area, the larger the bearing diameter the shorter can be its length and this in turn is likely to allow the distance between cylinders and therefore the overall length of the engine to be shorter, this having particular significance in the case of "V" form engines where two connecting rod big ends share a common crankpin. Secondly, in an axial view of the crankshaft, the greater the overlap between the diameters of the crankpins that carry the big end bearings and the diameters of the main crankshaft bearings, the greater the crankshaft strength. In the case of diesel engines where bearing loads are apt to be particularly heavy, there have been many examples of the facings between the connecting rod and its bearing cap being cut at an oblique angle to the longitudinal axis of the connecting rod. Whilst this produced a narrow connecting rod relative to the bearing diameter, it brought with it asymmetric stress distribution with a resultant distortion problems when under load, and the elaborate methods needed to locate the cap and prevent unduly high bending loads in the bolts resulted in added manufacturing cost and the risk of undesirable areas of stress concentration.
There is therefore considerable incentive to design big ends with a 90.degree. split (that is where the plane in which the facings between the connecting rod and the end cap lie is at 90.degree. to the longitudinal axis of the connecting rod), and various methods have been tried in attempts to optimise the size of big end bearing consistent with the ability to pass the big end of the connecting rod through the cylinder bore.
Known big end bearing designs having a 90.degree. split usually incorporate a two piece annular thin wall bearing shell of generally uniform thickness each semicircular half of this bearing shell having a pressed out "tang" for its axial location, the tang also providing a safeguard against rotation of the bearing shell within the big end. In such known designs the distance between the centre lines of the big end bolts on either side of the bearing is kept as small as possible without physically interfering with the bearing shell.
A further known development is to use four relatively small diameter big end bolts instead of two relatively large diameter bolts, thus enabling a larger bearing diameter to be arranged within the same overall outside dimension at the same time providing a more even clamping load on the bearing shells. When these bolt centres are reduced to the extent that the bolt either comes close to or interferes with the bearing shell, it becomes impracticable to find room for the usual tangs for bearing shell location purposes.